The present invention relates to an improved spincasting process for producing a series of contact lenses having desired shapes.
Spincasting (also termed centrifugal casting) is known in the art for curing polymerizable mixtures in the production of contact lenses. For example, spincasting methods are disclosed in U.S. Pat. Nos. 3,660,545 and 3,669,089. In conventional spincasting processes, a liquid polymerizable mixture is charged, generally via injection, to a mold having a concave molding surface, and the mixture is polymerized while the mold is rotated. The polymerizable mixture can be exposed to polymerizing radiation, such as ultraviolet radiation, and/or heated during rotation of the mold to facilitate polymerization. The produced lens has a convex lens surface shaped by the concave molding surface and a concave lens surface shaped by centrifugal force generated by rotation of the mold and surface tension of the mixture contained in the mold.
Polymerizable mixtures which may be used to form contact lenses are known to those of ordinary skill in the art. The mixtures comprise monomers or prepolymers, such as vinyl-containing monomers, which polymerize to form a polymer having desired properties such as optical transparency, strength and biocompatibility. The mixtures may also include other components known in the art such as crosslinking agents or polymerization initiators. Additionally, these mixtures may include a diluent or solvent, or the mixtures may be provided in bulk, i.e., without a diluent or solvent.
It is recognized in the art that the shape of the lens formed from the spincasting procedure is determined by various factors. As disclosed in U.S. Pat. No. 4,534,916, such factors include the size and the shape of the mold, the amount and the nature of the components in the lens-forming mixture, the rotational speed of the mold during polymerization, and the position of the axis of rotation of the mold relative to the direction of gravity. The shape of the convex lens surface is determined primarily by the mold design (and more particularly, by the shape of the molding surface of the molds in which the polymerizable mixtures are contained during spincasting), whereas each of the aforementioned factors can affect the shape of the concave lens surface formed from rotation of the mold.
Frequently, it is desired to produce contact lenses with differently shaped concave lens surfaces. More particularly, it is often desired to spincast such differently shaped contact lenses from the same mold design (i.e., molds having uniformly shaped concave molding surfaces) in order to minimize the need to employ a variety of different mold designs. In spincasting processes, this can be achieved by controlling the shape of the concave lens surface.
Conventionally, in spincasting processes for producing contact lenses with differently shaped concave lens surfaces from the same mold design, the same polymerizable mixture is maintained throughout the process. In other words, the polymerizable mixture remains fixed throughout the process, i.e., the individual polymerizable mixtures contain the same components, present in the same relative amounts. Accordingly, the shape of the concave lens surfaces of the produced lenses is varied by controlling factors such as volumes of the individual polymerizable mixtures or the rotational speed of the molds in which the individual mixtures are contained.
While the described conventional processes provides the production of contact lenses having differently shaped concave lens surfaces from the same mold design, the inventors of the present invention recognized several drawbacks to such conventional methods.
First, since the aforementioned factors which determine the shape of the concave lens surface are interrelated, when one of these factors is varied, such as volumes of the individual polymerizable mixtures, it may be necessary to compensate for this variation by adjusting other factors, such as the rotational speed or the extent and duration of exposure to polymerizing radiation. Otherwise, the polymerization process may yield an article which does not have a desired shape or which is not completely cured.
Second, production-scale spincasting apparatus require that an operator preliminarily set-up various operating conditions of the apparatus before initiating the spincasting operations. For example, an operator will set-up the apparatus based on operating conditions such as the rotational speed of the molds, the injection volume for injection of the liquid polymerizable mixtures into the molds, or the intensity or duration of ultraviolet radiation exposure. Subsequently, a series of contact lenses can be obtained by spincasting within the operating parameters. If one wishes to change such operating conditions, the spincasting operation may have to be ceased to allow for readjustment of the apparatus. Accordingly, the described conventional spincasting production processes, wherein contact lenses having differently shaped concave lens surfaces are produced, often require ceasing the spincasting operation in order to change the operating conditions of the apparatus.